The shape of Damaroo (Shiva's musical instrument) is symbolic representation of the relativistic spacetime light cone of past present and future (http://imgur.com/phgUBgS). This is consistently plausible with another belief that Shiva is called Mahakaal (Greater Time).

Sage Dhuracharya says...the wisdom is in knowing that one who has firm grasp on the present, can influence the past and present (rock both past and present).